As disclosed in Klaus Finkenzeller, “RFID-Handbuch, Grundlagen und praktische Anwendungen induktiver Funkanlagen, Transponder und kontaktloser Chipkarten”, 3rd edition, Hanser, Munich, 2002, one common method to transmit data from an RFID transponder or tag to a reader is load modulation, which is a special form of amplitude modulation. When being close to the reader, then the transponder is inductively coupled to the reader. The reader transmits a magnetic field and the transponder represents a load for the reader. By adjusting the load impedance of the transponder by, for instance, adjusting the load resistance or the capacitance of the transponder, the transformed impedance at the reader can be varied, resulting in a varying voltage across the antenna of the reader. Thus, load modulation is achieved.
Conventional RFID systems based on load modulation are limited by the achievable data rate and the ratio between the amplitude of the carrier signal and the modulated sequence.